Honestly, I don't know if the XP bonus on the mount {10%} lasts after RAF. It might, but either way, once you have 1 toon at max level {adventure or crafting} you get bonus xp for leveling all other toons on your account. I will say, however, the RAF mount is one of the fastest, if not the fastest, mount in the game. So if not for the xp, you should try to get the RAF mount to have the fast mount. Otherwise you'll be paying a lot for slower mounts.

I would venture to guess that the XP bonus on the mount will remain, as the level 80 guild mounts have stat bonuses. And being that is is an heirloom and you can trade the mount back and forth across all toons on the master account further makes me think that the mount will remain with the bonus so you can continue to mentor your friends. So even after RAF runs out, you will still have {probably} the mount bonus, the max-level toon bonus {10% per max level, up to 50%} and mentoring bonus to work with for subsequent teams.